Radon is an invisible, odorless and chemically inactive radioactive gas that is produced by the decay of uranium ore, such as radium, actinium, or thorium. As described in the introductory (background) portion of my U.S. Pat. No. 5,319,208, issued Jun. 7, 1994, entitled "Forced Air Flow Radon Detector Having Electrically Conductive Inlet and Exhaust Filters," during its decay process, radon produces several high energy alpha particles and solid, radioactive decay products, termed daughter products. In particular, Rn222 decays by emitting an alpha particle having an energy of 5.5 MeV (million electron volts) to produce a radioactive daughter ion Po218. Po218 then decays by emitting an alpha particle having an energy of 6.0 MeV to produce a radioactive daughter ion Po214. The Po214 ion subsequently decays by emitting an alpha particle having an energy of 7.7 MeV to produce radioactive daughter Po210. Po210, which has a half life of 20 years, eventually decays by emitting an alpha particle having an energy of 5.3 MeV.
Because inhaling radon and its radioactive decay products causes irradiation of lung tissue, prolonged exposure to high concentrations of radon significantly increases the risk of developing cancer. It has been reported that the U.S. Environmental Protection Agency estimates exposure to naturally occurring radon leads to 21,000 lung cancer deaths nationwide each year, making radon the nation's primary environmental health threat and second only to cigarette smoking as a cause of fatal lung cancer.
Although it was originally believed that dangerous levels of radon occurred primarily in uranium mines or laboratories having large quantities of uranium, various studies have indicated that radon produced by the decay of radioactive ore in the rock and soil migrates to the earth's surface and becomes trapped in residential buildings, where indoor concentrations of radon eventually build up to dangerous levels, thereby creating a significant residential health hazard. Indeed, indoor radon is now believed to be a greater radiological hazard to the general population than all other natural and man-made radiation sources combined. It has been estimated that between 6 and 9 million homes in the United States have radon levels above 4 pCi/1 (pico Curies per liter of air), the level above which the Environmental Protection Agency urges remedial action.
Whether the occupants of a building are at risk due to unacceptably high concentrations of radon can be determined only by actual measurement of air samples within the building. The tremendous volume of testing required to identify those buildings which are at risk has created a need for a radon gas detector which possesses the following characteristics. First the measurement must be reasonably fast. The instrument must also perform a high precision measurement. It should also be relatively low cost, in order to be practically affordable, and it should not require a skilled operator or the need for follow on laboratory analysis. In addition, the instrument should be capable of measuring radon concentration in pCi/1 or radon daughter product concentration in working level (WL) units, or both.
Various types of equipment and components have been proposed to date for radon detection. For example, an article by M. Wrenn et al, entitled: "Design of a Continuous Digital-Output Environmental Radon Monitor," Institute of Electrical and Electronics Engineers Trans. Nucl. Sci. NS-22:645-648, 1975, and an article by P. Hopke et al, entitled: "Use of Electrostatic Collection of Po-218 for Measuring," Rn. Health Physics, Vol. 57, No. 1 (July), pp. 39-42, 1989; describe the use of an electrostatic field for Po-218 collection. In particular, Wrenn et al describe placing a polyurethane foam radon daughter filter over a detector with a coarse metal screen over it, to form a positive (anode) electrode. A piece of aluminized mylar with phosphors on the surface is used as the negative electrode (cathode). Underneath the mylar is a photomultiplier tube that detects scintillations produced on the phosphor by alpha particles from the decay of deposited Po-218 ions.
An article by E. Albrecht et al, entitled: "Continuous Registration of Rn-222 Concentration in Air Varying with Time," in Assessment of Airborne Radioactivity in Nuclear Operations. Vienna, International Atomic Energy Agency, 1967 describes a device in which radon and its daughters were pumped into a separation chamber through a membrane to filter out the existing daughters. A surface barrier silicon photodiode was the detector with a gold metal barrier on the surface as the cathode of the electric field. The conductive inner surface of the separation chamber served as the anode. The photodiode was biased with 35 volts.
An article by A. C. George, entitled: "A Passive Environmental Radon Monitor," in Radon Workshop, February, 1977, New York: Health and Safety Laboratory; HASL-325, pp 25-30, 1977, describes the use of an electric filed to accumulate Po-218 ions onto a LiF crystal detector. This is a passive detector that stores beta and gamma radiation that can later be read out on a thermoluminescent dosimeter (TLD) analyzer. The radon daughter filter used was a paper filter. The anode device was an inverted metal funnel with a perforated metal disk at the large opening. The cathode was a bolt with the TLD cemented to it. This radon monitor is not a real time measuring device, since processing of the TLD is required after the measurement to determine radon concentration.
For descriptions of other proposals for radon detection, attention may be directed to an article by R. Miller, entitled: "Development of a Rapid Response Radon Monitor," Final report to Bureau of Mines, Denver, Colo., U.S. Bureau of Mines, Contract No. HO262019, 1979; an article by J. Porstendorfer et al, entitled: "Influence of Electric Charge and Humidity Upon the Diffusion Coefficient of Radon Decay Products," Health Phys. 15, pp191-199, 1979; an article by G. Keller et al entitled: "Method for the determination of Rn-222 (Radon) and Rn-230 (Thoron) Exhalation Rates Using Alpha Spectroscopy," Radiat. Prot. Dosim. 3(1/2), pp 83-89, 1982; an article by R. Washington et al, entitled: "The Measurement of Low Concentrations of Radon in Air," Health Phys 45, pp 559-561, 1983; an article by H. Tovedal, entitled: "Radon Measurement Activities and Instruments Designed at Studsvik Energiteknik," A. B. Radat Prot Dosim. 7(1-4), pp 215-218, 1984, an article by E. Pereira et al, entitled: "Radon-222 in the Antarctic Peninsula during 1986," Radiat. Prot. Dosim., 1989; an article by J. Ackers, entitled: "Direct Measurement of Radon Exhalation from Surfaces," Radiat. Prot. Dosim. 7(1-4), pp 199-201, 1984; and an article by S. Watnick et al, entitled: "Rn-222 Monitor Using a Spectroscopy," Health Phys. 50, pp 645-646, 1986.
Currently available radon detectors include scintillation and photomultiplier detectors, solid state junction and surface barrier photodiode detectors, gas proportional detectors, alpha track detectors, and charcoal canisters. However, none of these radon detectors has all of the features currently desired in a radon detector.
For example, Honeywell Inc. has marketed a device that uses a relatively simple and compact open photodiode detector to sense alpha particle emission. However, because its radon daughter filter porosity is only 0.8 microns, its response time is inordinately long. Another consideration in the design of a radon detector is the presence of electrical noise (always a problem in any electronic instrumentation).
In a radon measurement device, noise can be considered to be any undesirable electrical signal or pulse that can be interpreted as the desired pulse, thereby producing an error in the measurement. False pulses produced by electrical noise inflate the measurement of radon or produce a "background" pulse count, even in the absence of radon.
One attempt to solve the noise problem is described in the U.S. Patent to W. Simon et al, U.S. Pat. No. 4,871,914, entitled: "Low-Cost Radon Detector." The patented device describes the use of a dummy circuit with active elements (amplifier) parallel to the detector circuit, in order to cancel out transient noise and some microphonics. Photodiode detectors having special bonding of the leads without lead solder are employed avoid possible alpha emitting contaminants in lead. These measures require custom fabrication of the photodiode assemblies and many additional components including amplifiers, all adding to the cost of the instrument but not contributing to the reduction of all the noise components.
Advantageously, the device described in my above-referenced patent is able to provide the previously described characteristics that are desired of a radon device, without the drawbacks of other currently commercially available devices. For this purpose, my patented radon gas detector is comprised of a housing having an air inlet port leading to an interior, a radiation detection (e.g. alpha particle measurement) chamber and an air exhaust port leading from the interior chamber to the exterior of the housing. The interior chamber is closed to the entry of ambient light by means of a pair of light-obstructing baffle structures that respectively couple the air inlet and air exhaust ports to the interior chamber. The light obstructing baffle structure between the air inlet port and the interior chamber has an air passageway whose length is in excess of its widthwise dimension, so as to effectively prevent the entry of ambient light into the interior chamber by way of the air inlet port.
Coupled with the air inlet port is a first removable, electrically conductive mesh filter, through which air entering the air inlet port passes in the course of its movement to the interior chamber. Similarly, coupled with the air exhaust port is a second electrically conductive mesh filter. A significant feature of my patented device is fact that each electrically conductive mesh filter traps (ionic) Po-218 and Po-214 radon daughter products before they can enter into the interior chamber, while simultaneously allowing substantial air flow (through the openings in the mesh).
When air is drawn into the alpha particle measurement chamber under the control of a forced air movement device, such as an exhaust fan disposed in the air flow path through the exhaust port from the interior chamber, a substantial quantity of air per unit time can be actively circulated through the measurement chamber, thereby significantly reducing the length of time required to obtain a meaningful measurement of radon concentration within the ambient air under test. Disposed within the measurement chamber is a radiation (e.g. alpha particle responsive) detector in the form of an open photodiode array, which is exposed to incident alpha particle emissions from the radon gas as it is drawn through the interior chamber by the operation of the exhaust fan.